Short circuit protection device for multiple power supplies

ABSTRACT

A short circuit protection device includes a surge protection circuit having first and second input connections coupled to a voltage source and a control circuit, respectively, and an output connection coupled to a plurality of voltage regulator circuits. The output voltages of the regulator circuit are sensed through a gate circuit by the control circuit. In the event one of the regulator output voltages goes to zero, the control circuit directs a control signal to the surge protection circuit which then disconnects the voltage source from the plurality of regulator circuits.

United States Patent Stefani 1451 May-23, 1972 [54] SHORT CIRCUITPROTECTION 3,303,386 2/1967 Murphy ..317/33 VR 3,582,713 6/1971 Till.317/33 VR DEVICE FOR MULTIPLE POWER SUPPLIES Primary Examiner-James D.Trammell Attorney-Norman J. O'Malley, Elmer J. Nealon and Robert T. Omer[57] ABSTRACT A short circuit protection device includes a surgeprotection circuit having first and second input connections coupled toa voltage source and a control circuit, respectively, and an outputconnection coupled to a plurality of voltage regulator circuits. Theoutput voltages of the regulator circuit are sensed through a gatecircuit by the control circuit. In the event one of the regulator outputvoltages goes to zero, the control circuit directs a control signal tothe surge protection circuit which then disconnects the voltage sourcefrom the plurality of regulator circuits.

[72] Inventor: Richard D. Stefani, Le Roy, NY.

[73] Ass'ignee: GTE Sylvanla Incorporated [221 Filed: Dec. 2, 1970 [21]Appl. No.: 94,376

[52] US. Cl ..3l7/22, 317/31, 317/33 VR, 323/22 T [51] Int. Cl. ..H02h7/20 [58] Field of Search ..317/33 VR, 22, 36 TD, 38, 50, 317/31; 323/22T [56] References Cited UNITED STATES PATENTS 3,229,164 l/1966 McCartneyetal ..317/33 VR CONTROL CIRCUIT FROM VOLTAGE w SOURCE 12 POWER LINE SisPatented May 23, 1972 2 Sheets-Sheet l 18 VOLT VOLTAGE SOURCE V i I i Il lllll .ll !|I h v V R R R O O O W m M U J W M V U E M .6% 8 5% m R R RIII I I I I II I |.l 1 V H N ll-lllll m 1 k mm T l I l I L E m w m v 1 6INVENTOR.

RICHARD D. STEFAN! BYg (j Patented May 23, 1972 2 Sheets-Sheet 2 TOREGULATORS l8o--|8c 2 Q6 &

iii/l2 SURGE PROTECTION CIRCUIT FIG. 2

CONTROL c|Rcu\T/ LINE FROM VOLTAGE SOURCE IZ/ POWER INVENTOR.

RICHARD D. STEFAN! B v V) ATTORNEY BACKGROUND OF-THE INVENTION Thisinvention relates to protection devices for electronic circuitry and inparticular to a protection device for protecting a primary power sourceagainst short circuit the base and voltage surge conditions externalthereto.

Current limiting or short circuit protection is typically accomplishedby connecting a resistor to the emitter of a seriespass transistor andby connecting a diode between the base of the transistor and the lowvoltage end of the resistor. The diode clamps the voltage across theresistor thereby limiting the current supplied by the transistor. When ashort circuit occurs, the collector to emitter voltage drop V increasesthereby causing the power dissipated within the pass transistor toincrease greatly. The inherent disadvantages of the increased powerdissipation are obvious.

The dissipation problem is further compounded in a system which requiresa plurality of regulated voltage supplies, each of which must have shortcircuit protection. It would therefore be advantageous to have, and itis one of the objects of this invention to provide, a low powerdissipation device for protecting a prime power source when a shortcircuit occurs in any one of a plurality of regulated voltage suppliescoupled to the prime power source.

BRIEF SUMMARY OF THE INVENTION A device according to the presentinvention for protecting a power source includes a surge protectioncircuit having an input connection coupled to the power source, anoutput connection coupled to a load, and a control terminal and isoperative to couple-the power source to the load. A voltage sensingmeans has an input connection coupled to the load and an outputconnection coupled to the control terminal of the surge protectioncircuit and is operative to direct a control signal to the surgeprotection circuit when the voltage at the load falls below apredetermined value. The surge protection circuit is operative todecouple the load from the power source in response to either a controlsignal at its control terminal or a surge at its input connection.

BRIEF DESCRIPTION OF THE DRAWINGS The construction and operation of adevice according to the present invention for protecting a power sourceagainst a short circuit condition will be more fully understood from thefollowing detailed description taken in conjunction with the ac-DETAILED DESCRIPTION OF THE INVENTION An embodiment of a deviceaccording to the present invention for protecting a power source isshown in block diagram form in FIG. 1 and includes a surge protectioncircuit 10 (to be discussed in detail hereinafter) having an inputconnection coupled to a voltage source 12, such as any well-known directcurrent power supply, a control terminal coupled to a sensing means 13including a control circuit 14 and a gate 15 and an output connectioncoupled to a load 16. The load 16 may, for example, include a pluralityof voltage regulator circuits l8a-18 (three voltage regulator circuitsare shown by way of example). The gate 15 has input connections fromeach of voltage regulator circuits l8a-18c and an output connectionconnected to the control circuit 14 (to be discussed in detailhereinafter).

Under quiescent operating conditions, prime power is supplied to thevoltage regulators 18a-18c from the voltage source 12 via the surgeprotection circuit 10. The output voltages of the voltage regulators18a-18c are sensed by the gate 15 (well-known in the art) whichmaintains a high output, for

example 5.0 volts, when all three voltages are present at its inputconnections and drops to zero volts when any one of the input voltagesgoes to zero. In response to the zero volt level of the gate 15, thecontrol circuit 14 directs a signal to the surge protection circuit 10which, in turn, disconnects the load 16 from the voltage source 12.

A schematic diagram of an embodiment of the control circuit and surgeprotection circuit is shown in FIG. 2. The control circuit 14 includes acurrent limiting resistor R1 and an isolating diode CR1 connectedbetween the gate 15 and the base of a first transistor Q1 (for example,an NPN type), the emitter of which is connected to a source of referencepotential such as ground and the collector of which is connected througha load resistor R2 to the voltage source 12. A series combination of afeedback capacitor c1 and a zener diode CR2 is connected between thebase and collector of the first transistor Q1.

A series combination of a second zener diode CR3 and a second isolatingdiode CR4 is connected between the base of the first transistor Q1 andthe collector of a second transistor Q2 (for example, an NPN type), theemitter of which is grounded and the base of which is connected througha second limiting resistor R3 to the collector of the first transistorQ1. A series combination of a third zener diode CR5 and a thirdisolating diode CR6 is connected between the collector of the firsttransistor to the base of a third transistor Q3 (for example, an NPNtype) of the surge protection circuit 10.

A series combination of a fourth resistor R4, a fourth zener diode CR7and a fifth resistor R5 is connected between the voltage source 12 andground. The base of the third transistor Q3 is connected to the commonjuncture of the fifth resistor R5 and the fourth zener diode CR7, theemitter is connected to ground, and the collector is connected through asixth resistor R6 to the base of a fourth transistor Q4 (for example, aPNP type). A series combination of a fifth zener diode CR8, a seventhresistor R7 and an eighth resistor R8 is connected between the base ofthe third transistor Q3 and ground. Connected between the commonjuncture of the seventh and eighth resistors R7 and R8 and the collectorof the second transistor Q2 is ninth resistor R9.

Also connected to the common juncture of the seventh and eighthresistors is the collector of the fourth transistor Q4, the emitter ofwhich is connected to its base through a tenth resistor R10. The emitterof the fourth transistor Q4 is also connected to the base of a fifthtransistor Q5 (for example, a PNP type), the emitter of which isconnected to the voltage source 12 and the collector of which isconnected through a fourth isolating diode CR9 to the common juncture ofthe base of a sixth transistor Q6 (for example, a PNP type) and aneleventh resistor R11. The emitter of the sixth transistor Q6 and theother end of the eleventh resistor R11 are connected to the voltagesource 12 while the collector of the sixth transistor Q6 is connected tothe regulators 18a-18c.

Under normal quiescent operating conditions, current flows from the 5.0volt line (from the gate 15) through the first resistor R1 and the firstisolating diode CR1 forcing the first transistor O1 to saturate. Nocurrent flows through zener diode CR5 and the isolating diode CR6, andcurrent is supplied from the source 12 to the regulators 1841- via powerline 11 and the series transistor Q6. The second, third, fourth andfifth transistors Q2 through Q5 are normally off while the sixthtransistor is normally on allowing current to flow from the voltagesource 12 to the regulators l8a-18c.

When any one of the output power lines from the regulators l8a-18 isshorted to ground, the gate 15 changes state and the +5.0 volts isremoved from the base circuit of the first transistor Q1. The firsttransistor O1 is thus turned off allowing current to flow through thezener diode CR5 and the isolating diode CR6 to the surge protectioncircuit 10 which removes the regulators 18a-18c from the voltage source12 in a manner to be discussed hereinbelow. During the short circuitcondition, current also flows through the third resistor R3 to the baseof the second transistor Q2 causing the transistor Q2 to saturate. Nocurrent flows through the zener diode CR3 forcing the first transistorQ1 to remain off. The zener diode CR2 allows the second transistor Q2 tosaturate immediately after a short circuit condition exists by allowingthe voltage at the collector of the transistor Q1 to rise immediately tothe voltage of the zener diode CR2 before the capacitor C1 begins tocharge. This forces current to flow to the base of the transistor Q2.

The capacitor C1 assures that the control circuit 14 will turn on whenprimary power is first applied by forcing the collector of the firsttransistor Q1 to remain below a predetermined voltage level, forexample, 6.8 volts, until the 5.0 volt line is established. When the 5.0volt line is established, the first transistor Q1 saturates and normaloperation is again resumed.

The surge protection circuit operates as follows. The control transistorQ6 is normally biased into saturation (the base current to saturate thecontrol transistor Q6 can provide the necessary current to operate oneof the regulator circuits). Circuit protection from either a power linesurge or a regulator short circuit condition is achieved by turning thecontrol transistor Q6 off.

When a power line transient raises the voltage at the cathode of thezener diode CR7 to a predetermined level, for example 33.0 volts, thecontrol transistor O6 is turned off in the following manner. Currentflows through the fourth resistor R4 and the zener diode CR7 to the baseof the third transistor Q3 which is turned on allowing current to flowfrom the power line 11 through the emitter-base junctions of the fifthand fourth transistors Q5 and Q4. The current is sufficient to saturatethe fourth and fifth transistors Q4 and Q5. The saturation of the fifthtransistor stops the current flow in the emitter-base junction of thecontrol transistor Q6 which, in turn, disconnects the regulator circuits18a-18c from the power line 11.

During a surge condition, current flows from the collector of thetransistor Q4, the seventh resistor R7 and the zener diode CR8 to thebase of the transistor Q3 (the function of the resistor R7 and the zenerdiode CR8 will be explained hereinafter). Current also flows from thecollector of the transistor Q4 through the resistor R9, the isolatingdiode CR4 and the zener diode CR3 to the base of the transistor Q1. Thusthe first transistor Q1 remains saturated during the power line surgeduration even though the voltage from the gate is absent. Thus the firsttransistor circuit is inhibited from generating a signal at the base oftransistor Q3.

The condition of the circuit remains as described above until the powerline surge drops below the knee of the zener diode CR7 (=33.0 volts).While current stops flowing through the zener diode CR7, it continues toflow through the zener diode CR8 and the transistor Q3 remains saturateduntil the surge drops below the knee of the zener diode CR8 (-30.0volts). Current stops flowing through the zener diode CR8 and thetransistor Q3 is turned off. The fourth and fifth transistors Q4 and Q5turn ofi allowing base current to flow through the control transistor Q6and normal operation is again resumed.

There is a time delay between the time the transistor Q4 turns off andthe time that the gate voltage from gate 15 is reestablished. When thetransistor Q4 turns off, current stops flowing through the zener diodeCR3 and the transistor Q1 is turned off. The capacitor C1 slows down theturn-off time of the transistor Q1. The collector of the transistor Q1remains below 6.8 volts during the time that the '+5 .0 volt line fromthe gate is being established. Once established, current again flowsthrough the resistor R1 and the diode CR1 to the base of the transistorQ1 which then saturates. If the transistor Q1 were allowed to turn offimmediately after the transistor Q4 turns off, current would flowthrough the resistor R2, the zener diode CR5 and the diode CR6 to thebase of the transistor Q3. The transistor Q3 would then saturate turningoff the transistor Q6 again.

(=30 volts) than the deactivation voltage thus preventing the smalltransients on the power line from reactivating the surge protectorcircuit 10. When transistor Q6 saturates, after a power line surgecondition, an in-rush of current comes from the power line through thetransistor O6 to regulator circuits 18a-18. The current in-rush iscaused by charging of the capacitors associated with the regulators.Once the capacitors (not shown) are charged, the current drops to thenormal load value.

Current, under quiescent conditions, flows from the +5.0

volts line through the resistor R1 and the isolating diode CR1 forcingthe transistor Q1 to saturate. If any of the regulators 18a-l8 havetheir output lines shorted to ground, the +50 volt line drops to zerocausing the transistor Q1 to turn off and allowing current to flowthrough the zener diode CR5 and the isolating diode CR6 to the base ofthe transistor Q3. The surge protection circuit 10 is then activated inthe manner described hereinabove. The zener diode CR2 allows current toflow, immediately after a short circuit condition exists, from thecollector of the transistor Q1 through the resistor R3 to the base ofthe transistor Q2. The transistor Q2 saturates and prevents current fromflowing through the zener diode CR3.

While there has been shown and described what is considered a preferredembodiment of the present invention, various modifications may be madetherein without departing from the invention as defined by the appendedclaims.

What is claimed is: l. A device for protecting a power sourcecomprising: a load having an output voltage associated therewith; asurge protection circuit having an input connection cou pled to saidpower source, an output connection coupled to said load, and a controlterminal and being operative to couple energy from said power source tosaid load; and voltage sensing means having an input connection coupledto said load and an output connection coupled to the control terminal ofsaid surge protection circuit and being operative to direct a controlsignal to said control terminal when the output voltage at said loadfalls below a predetermined value; said surge protection circuit beingoperative in response to the control signal at its control terminal fromsaid voltage sensing means to decouple said load from said power sourceand being operative in response to a surge condition at its inputconnection to decouple said load from said power source; said voltagesensing means including means having an input connection coupled to saidload and an output connection and being operative to generate an outputsignal at its output connection when an output voltage less than apredetermined level occurs at the input connection, and

first transistor circuit means having an input connection connected tothe output connection of said lastmentioned means and an outputconnection coupled to the control terminal of said surge protectioncircuit and being operative in response to the output signal from saidlast-mentioned means to generate said control signal;

means coupled between said surge protection circuit and the inputconnection of said first transistor circuit means, for rendering saidfirst transistor circuit means nonresponsive to a signal from said surgeprotection circuit during said surge condition; and

second transistor circuit means having an input connection and an outputconnection connected to the output connection and input connection,respectively, of said first transistor circuit means and being operativeto render said first transistor circuit means nonresponsive to signalsat its input connection after said control signal is generated by saidfirst transistor.

2. A device for protection a power source according to claim 1 wherein:

said load includes a plurality of regulated voltage supplies each havingan input connection coupled to said surge protection circuit and anoutput connection coupled to said voltage sensing means and beingoperative to generate a regulated output voltage; and said means in saidvoltage sensing means includes gating means having an input connectioncoupled to each of said regulated supplies and an output connectionconnected to the input connection of the first transistor circuit meansand being operative to generate an output signal at its outputconnection if any one of the output voltages is less than apredetermined level. 3. A device for protecting a power source accordingto claim 2 wherein said surge protection circuit includes:

third transistor circuit means, coupled between said power source andsaid plurality of regulated voltage supplies, having a control elementand being operative in response to a control signal at its controlelement to decouple said power source from said plurality of regulatedvoltage supplies; fourth transistor circuit means having an inputconnection coupled to said power source and to said control terminal andan output connection and being operative in response to either a surgeat said power source or a signal from said first transistor circuitmeans to change from a first condition to a second condition at itsoutput connection; and fifth transistor circuit means having an inputconnection coupled to the output connection of said fourth transistorand an output connection coupled to the control element of said thirdtransistor circuit means and being operative in response to the secondcondition of said fourth transistor circuit means to generate saidcontrol signal at the control element of said third transistor circuitmeans. 4. A device for protecting a power source according to claim 3further including a sixth transistor circuit means having an inputconnection coupled to the output connection of said fourth transistorcircuit means and an output connection coupled to the input connectionof said first transistor means and being operative in response to thesecond condition at the output connection of said fourth transistorcircuit means caused by the surge at said power source to inhibit thegeneration of a control signal at said control terminal.

1. A device for protecting a power source comprising: a load having anoutput voltage associated therewith; a surge protection circuit havingan input connection coupled to said power source, an output connectioncoupled to said load, and a control terminal and being operative tocouple energy from said power source to said load; and voltage sensingmeans having an input connection coUpled to said load and an outputconnection coupled to the control terminal of said surge protectioncircuit and being operative to direct a control signal to said controlterminal when the output voltage at said load falls below apredetermined value; said surge protection circuit being operative inresponse to the control signal at its control terminal from said voltagesensing means to decouple said load from said power source and beingoperative in response to a surge condition at its input connection todecouple said load from said power source; said voltage sensing meansincluding means having an input connection coupled to said load and anoutput connection and being operative to generate an output signal atits output connection when an output voltage less than a predeterminedlevel occurs at the input connection, and first transistor circuit meanshaving an input connection connected to the output connection of saidlast-mentioned means and an output connection coupled to the controlterminal of said surge protection circuit and being operative inresponse to the output signal from said last-mentioned means to generatesaid control signal; means coupled between said surge protection circuitand the input connection of said first transistor circuit means, forrendering said first transistor circuit means nonresponsive to a signalfrom said surge protection circuit during said surge condition; andsecond transistor circuit means having an input connection and an outputconnection connected to the output connection and input connection,respectively, of said first transistor circuit means and being operativeto render said first transistor circuit means nonresponsive to signalsat its input connection after said control signal is generated by saidfirst transistor.
 2. A device for protection a power source according toclaim 1 wherein: said load includes a plurality of regulated voltagesupplies each having an input connection coupled to said surgeprotection circuit and an output connection coupled to said voltagesensing means and being operative to generate a regulated outputvoltage; and said means in said voltage sensing means includes gatingmeans having an input connection coupled to each of said regulatedsupplies and an output connection connected to the input connection ofthe first transistor circuit means and being operative to generate anoutput signal at its output connection if any one of the output voltagesis less than a predetermined level.
 3. A device for protecting a powersource according to claim 2 wherein said surge protection circuitincludes: third transistor circuit means, coupled between said powersource and said plurality of regulated voltage supplies, having acontrol element and being operative in response to a control signal atits control element to decouple said power source from said plurality ofregulated voltage supplies; fourth transistor circuit means having aninput connection coupled to said power source and to said controlterminal and an output connection and being operative in response toeither a surge at said power source or a signal from said firsttransistor circuit means to change from a first condition to a secondcondition at its output connection; and fifth transistor circuit meanshaving an input connection coupled to the output connection of saidfourth transistor and an output connection coupled to the controlelement of said third transistor circuit means and being operative inresponse to the second condition of said fourth transistor circuit meansto generate said control signal at the control element of said thirdtransistor circuit means.
 4. A device for protecting a power sourceaccording to claim 3 further including a sixth transistor circuit meanshaving an input connection coupled to the output connection of saidfourth transistor circuit means and an output connection coupled to theinput connection of said first transistor means and being operative inresponse to the second condition at the output connection of said fourthtransistor circuit means caused by the surge at said power source toinhibit the generation of a control signal at said control terminal.